In the last few years, numerous developments have become known which deal with improving the mono- or multilayer filter bags made of paper or paper and tissue which have been known for a long time in the state of the art. DE 3 812 849 describes dust filter bags with a filter paper outer layer and an internally situated melt-spun microfibre nonwoven fabric (melt-blown). A multilayer filter bag made of nonwovens (SMS) is disclosed for example in U.S. Pat. No. 4,589,894 and U.S. Pat. No. 5,647,881. These inventions deal mainly with improving the dust separation. In EP 1 258 277 B1 and EP 0 960 645 B1, combinations of nonwovens are described which have a particularly long service life and good dust separation capacity. EP 1 362 627 A1 describes filter bags with a multilayer construction in which the fibre diameter distributions in the coarse dust filter layer and in the fine dust filter layer have gradients. In EP 1 254 693 A2, a vacuum cleaner bag is described in which a pre-filter layer made of a dry-laid, electrostatically effective nonwoven is present in front of a filter layer.
Furthermore, a filter medium made of a film fibre nonwoven is described in EP 1 197 252 A1, which comprises dry-laid, electrostatically charged split fibres which are connected to each other by ultrasonic welding. In order to achieve sufficient strength of the nonwoven, it is essential that at least two ultrasonic weld points per cm2 are present. It is achieved as a result that the individual fibres are connected directly to each other by ultrasonic welds. It is mentioned as an advantage of such a filter medium that the manufacturing rate in comparison with needle bonding of the fibre web with a scrim is higher and the air resistance of the scrim can be avoided. In the above-mentioned application, it is also provided furthermore that the film fibre nonwoven can also be connected to further nonwoven fabric layers. It is disadvantageous with the filter medium of EP 1 197 252 A1 above all that the dust storage capacity of this material is inadequate for use as filter medium for vacuum cleaner bags.